For years, extrusion dies (also known as extruders) have been provided with various mechanical means for adjusting the die lip gap in order to provide a film or sheet with a controlled, and adjustable, thickness. Typically, the lip of the extruder includes a degree of flexibility, often from a slideable portion. In some older designs, a plurality of bolts spaced apart from each other along the lip control the lip gap along the lip. Adjusting the individual lip bolts involves considerable skill, takes time and adds to the cost of production.
Use of thermal expansion and contraction of lip-adjusting components has been described in various references; see for example, U.S. Pat. No. 3,940,221 to Nissel, U.S. Pat. No. 4,753,587 to Djordjevic, et al, and U.S. Pat. No. 5,208,047 to Cloeren, et al. Newer approaches for adjusting the lip gap include using an adjustable, hinged lip; various examples are described in U.S. Pat. Nos. 5,679,383 and 5,962,041 to Ryan et al., U.S. Pat. No. 6,017,207 to Druschel, and U.S. Pat. No. 6,287,105 to Druschel et al.
The approach of Ryan et al. is to use a generally laterally movable block or bar, which extends laterally across the extruder parallel with the lip, the block having one or more angled cam blocks extending therefrom. A lip adjustment block is coupled to the lip to be moved (usually the bottom lip) which includes one or more angled grooves or recesses sized to closely receive the cam blocks for sliding movement therein. A threaded stud extends axially from one end of the angle block through a hole in an end plate and is received by a complementarily threaded spool. Rotation of the spool results in generally lateral movement of the angle block and in single point adjustment of the lip gap.
This approach may solve many problems of conventional dies, however, the lip adjustment block and the corresponding angle block are difficult to manufacture. Because the cam blocks of the angle block fit into the angled grooves of the block and the entire lengths must fit perfectly, accurate machining is required. This difficulty increases with increased length. Likewise, replacement and repair of these components is expensive and time consuming, and because of the interfit, it may be necessary to remove both components.
In addition, the lengths of the lip adjustment block and angle block cause other problems. A lengthy component may tend to warp, and because of thermal expansion and retraction, the possibility of warping is increased. Moreover, long pieces (e.g., of sixty inches or more) usually require greater machining clearance for the pieces to mate properly, which may result in non-uniformity along the die lip. Furthermore, rotation of the spool can be difficult if the long pieces do not properly fit and align. If so, costly oversized or special thread forms may be necessary.
The improved approach of the Druschel and Druschel et al. references is to also use a generally laterally movable block or bar (referred to as an actuator bar), which extends laterally across the extruder parallel with the lip. This bar includes a plurality of pins attached thereto, which engage with angled slots in perpendicularly arranged slideable blocks. Multiple blocks slideably engage the lip. As the actuator bar is slid, each pin is slid within the slot, thus moving the slideable block in a direction transverse to the direction of movement of the actuator bar. A stud extends axially from one end of the actuator bar through a hole in an end plate. A spool, threaded over the stud, rotates, thus moving the stud with a generally linear movement that in turn moves the actuator bar and provides single point adjustment of the lip gap. Druschel et al. modify the adjustment mechanism by providing a geared assembly remote from, but connected to the extruder, to facilitate rotation of a gear connected to a stud, in order to provide linear movement of the actuator bar.
There is always room for improvements.